It's going to be a bimetallic strip. Two pieces of different metal sandwiched together, with different coefficients of thermal expansion. As the temperature changes, the two metals push and pull on each other, allowing the piece to move. Make this piece into a coil (the coil seen in the bottom-left corner), and it will curl and uncurl itself as temperature changes. We harness this movement to connect or disconnect circuits. You can find some great info on it from Technology Connections on youtube.

Chances are, the bimetallic strip has either broken or gotten stuck somehow.